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Tolerance settings

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I have tried to print a few threads that were designed in Fusion 360. The thread size is M6 and 1/4" which is needed to mate a 3D scanner to a tripod.

I have printed thread before using Ideamaker and slic3r and both give me a good fitting thread.

I would like to use Cura as my regular slicer, but everytime I do threads using Cura they come out too small. Cura is a great program with many options but I am still new to 3D printing and I have not been able to find the best settings to achieve repeatable tolerance.

I have printed, on Cura, calibration cubes ( no holes or threads) and they are good.

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If you print a cylinder vertically or a cylindrical hole vertically they come out smaller than desired. Especially the holes. This is because PLA comes out like a liquid rubber band - like snot - it cools rapidly and shrinks and is stretchy and sticks to itself like snot. Combine these properties and it's like a liquid rubber band.

So as it gets laid down around a vertical cylindrical hole, it pulls inward and the diamter of the hole is typically 0.4mm smaller than desired. Because of this I usually add 0.4mm to 0.5mm to every vertical hole diameter (half that if radius) in CAD.

Solid cylinders are also smaller but not by as much if the outer layer goes on last because the inner layers help support. I'm not sure how much they shrink - maybe only 0.1mm.

Basically if you are printing a nut I would look at the diameter of the hole and scale up X and Y (but not Z) by the right amount to add 0.5mm. Or you can set horizontal expansion to -0.5. That shrinks the outside of the part and enlarges vertical holes.

If you are printing a screw I would probably leave it as is.

When you need really precise parts turn off acceleration control and jerk control and set all the printing speeds the same (anytime you speed up or slow down you get some under or over extrusion) and set the print speeds pretty low - maybe 25mm/sec if you want a perfect part. Patience is rewarded.

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Thank you very much for the suggestions. I will try a few of them and see if that resolves my problem.

Just a couple of comments.

I am trying to understand why Cura does not create the same result as Slic3r and Ideamaker with regards to threads. Purchasing a M6 bolt does not help because I need an adaptor to go the a 1/4" bolt to a m6 thread.

Cura has many options and I am sure that once I undersatnd what setting needs to be changed, Cura will be my favorite slicer. But todya I use all 3 three for differnet slicing needs.

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Well if Cura is resulting in worse results than other slicers it's almost always temperature or printing speed. Set *all* of cura's printing speeds to the same value and match it to another slicer and I think the results will be closer (and disable jerk/accel control).

Also I recommend trying Cura 15.04.06. If you are going to compare cura to two other slicers, please try 15.04.06 as well. It's the same engine but somehow it gives me better results (although the difference is small enough that I don't really use it anymore).